The present invention relates to a driveline system for a four wheel drive vehicle, and more particularly, to a center differential device for use in such a driveline system. The use of the term "differential" does not imply the presence of conventional differential gearing, but instead, the term is used primarily because the device of the present invention replaces the typical prior art center differential. However, it should be understood that the device illustrated, described and claimed herein has uses other than as a center differential.
In many of the vehicles being produced today, the basic vehicle platform is a front wheel drive. However, in many such vehicles, especially in vans and sport utility vehicles, it is considered desirable to be able to provide four wheel drive, at least under certain operating conditions.
Typically, the various arrangements for achieving part-time four wheel drive have been complex and expensive, and in some cases, have required some sort of control scheme to achieve the four wheel drive (or rear wheel drive) in response to certain predetermined operating conditions.
In certain vehicle applications, it is acceptable to provide only front wheel drive under most operating conditions, and rear wheel drive is required only when the front drive wheels are slipping (e.g., under poor traction conditions). However, the prior art has not provided a suitable device wherein only front wheel drive is provided, with no substantial torque being transmitted to the rear wheels, until the front wheels begin to slip.
For example, U.S. Pat. No. 4,562,897 discloses a viscous clutch in the driveline, between the front transaxle and the rear wheels, but requires a selector arrangement in order to choose among transmitting torque through the viscous coupling, or disconnecting the viscous coupling, or bypassing the viscous coupling. Implementation of such a selector arrangement adds substantially to the overall complexity and cost of the driveline system.
It is known from U.S. Pat. No. 5,070,975 and from European Application EP 0 314 420 to use, as a center differential in a four wheel driveline, a viscous actuated ball ramp type friction clutch to transmit torque to the rear axles when there is a speed difference between the front and rear wheels. However, the devices of the cited patent and application have numerous functional problems, stemming in part from the attempt to "incorporate" one ramp plate of the ball ramp actuator into the viscous coupling. For example, rotating seals would be required to separate the viscous fluid from the fluid in the remainder of the device, which is generally considered undesirable from the standpoint of durability.
In addition, it is believed that the torque transmitting capability of the device of the cited patent and application would be somewhat limited by the fact that one of the ball ramp plates comprises the input to the viscous coupling. Finally, this relationship of the ball ramp and the viscous coupling, as described above, would increase the hysteresis of the device as well as the overall friction in the actuator portion of the device, thus adversely affecting the predictability of operation of the device.